Conventionally, with an engine in which high pressure is required for fuel that is supplied to an injector, such as with an in-cylinder direct injection engine for example, fuel that has been pumped from a fuel tank is compressed by a high pressure fuel pump, and then supplied to the injector.
Specifically, as disclosed in Patent Literature 1 below as well, a fuel supply system in this type of engine is configured so as to include a feed pump that pumps out fuel from the fuel tank, and a high pressure fuel pump that compresses the fuel that has been pumped out by the feed pump. Then, the fuel that has been compressed by the high pressure fuel pump is retained in a delivery pipe that is connected to a plurality of injectors. Accordingly, along with an opening operation of the injectors, the high pressure fuel retained in the delivery pipe is ejected from the open injectors toward a combustion chamber.
Also, the high pressure fuel pump included in the fuel supply system of this type of engine includes a plunger that reciprocates in a cylinder, a compression chamber that is defined by the plunger and the cylinder, and a discharge valve (check valve) arranged on the discharge side of the compression chamber. The volume of the compression chamber changes due to the reciprocation of the plunger in the cylinder, and thus fuel is taken into the compression chamber when the volume expands, and at a predetermined timing when the volume contracts, the discharge value is released and high pressure fuel is pumped toward the delivery pipe.
More specifically, the high pressure fuel pump is provided with an electromagnetic spill valve that opens and blocks off communication between the compression chamber and a low pressure fuel pipe on the intake side thereof, and in the compression stroke, the volume of the compression chamber is reduced due to the movement of the plunger in the cylinder. Then, while the electromagnetic spill valve is open during the compression stroke, fuel flows out of the compression chamber to the low pressure fuel pipe (flows out to the feed pump side), and therefore fuel is not pumped toward the delivery pipe. In contrast, when the electromagnetic spill valve is closed during the compression stroke, the pressure (fuel pressure) in the compression chamber rises, the discharge valve starts the opening operation when the pressure exceeds a resultant force obtained by adding together the biasing force of a coil spring that causes the valve element of the discharge valve to be biased in the closed direction and the fuel pressure in the delivery pipe, and fuel is pumped toward the delivery pipe during the closed period of the electromagnetic spill valve. In this way, the amount of fuel that is pumped from the high pressure fuel pump to the delivery pump is adjusted by controlling the closed period of the electromagnetic spill valve during the compression stroke.
With a fuel supply system that includes this type of high pressure fuel pump, when the engine has stopped, there is a high possibility that the internal pressure in the delivery pipe is in a high state since high pressure fuel had been pumped toward the delivery pipe by the high pressure fuel pump up to that time. Then, in a situation in which the internal pressure in the delivery pipe is maintained in the high state while the engine is stopped, it is possible for fuel to leak from the injection opening of the injector into the cylinder that the injection opening of the injector faces, as a result of, for example, the increase in the difference between the pressure in the space inside the injector on which the internal pressure of the delivery pipe acts and the internal pressure of the cylinder. In such a situation, there is concern that the presence of fuel that has leaked into the cylinder will adversely affect the next instance of engine starting.
In view of this point, in for example Patent Literature 2 and Patent Literature 3 below, a micropore is formed in the check valve arranged on the discharge side of the compression chamber, and after the engine has been stopped, fuel gradually returns to the high pressure fuel pump side through the micropore, which reduces the internal pressure in the delivery pipe, thereby preventing the leakage of fuel from the injector.